Many biological processes, including cell division, growth, and motility, rely on rapid remodeling of the actin cytoskeleton and on actin-filament severing by the regulatory protein cofilin. Phosphorylation of vertebrate cofilin at serine 3 (Ser-3) regulates both actin binding and severing. Substitution of serine with aspartate at position 3 (S3D) is widely used to mimic cofilin phosphorylation in cells and in vitro. The S3D substitution weakens cofilin binding to filaments, and it is presumed that subsequent reduction in cofilin occupancy inhibits filament severing, but this hypothesis has remained untested. Here, using time-resolved phosphorescence anisotropy, electron cryomicroscopy, and all-atom molecular dynamics simulations, we show that S3D-cofilin indeed binds filaments with lower affinity, but also with a higher cooperativity than wild-type cofilin, and severs actin weakly across a broad range of occupancies. We found that three factors contribute to the severing deficiency of S3D-cofilin. First, the high cooperativity of S3D-cofilin generates fewer boundaries between bare and decorated actin segments where severing occurs preferentially. Second, S3D-cofilin only weakly alters filament bending and twisting dynamics and therefore does not introduce the mechanical discontinuities required for efficient filament severing at boundaries. Third, Ser-3 modification (i.e. substitution with Asp or phosphorylation) undocks and repositions the cofilin N-terminus away from the filament axis, which compromises S3D-cofilins ability to weaken longitudinal filament subunit interactions. Collectively, our results demonstrate that, in addition to inhibiting actin binding, Ser-3 modification favors formation of a cofilin binding mode that is unable to sufficiently alter filament mechanical properties and promote severing.